Selected Correspondence:
Letter 40 (35) Spinoza to [the Elwes translation shows this as Christian Huyghens, Shirley and others show it as John Hudde.]

Distinguished Sir, --In your last letter, written on March
30th, you have excellently elucidated the point, which was
somewhat obscure to me in your letter of February 10th. As I
now know your opinion, I will set forth the state of the question
as you conceive it; whether there be only a single Being who
subsists by his own sufficiency or force? I not only affirm this
to be so, but also undertake to prove it from the fact, that the
nature of such a Being necessarily involves
existence; perhaps
it may also be readily proved from the
understanding of
God (as
I set forth, "Principles of Cartesian Philosophy," I. Prop. i.), or
from others of His attributes. Before treating of the subject I
will briefly show, as preliminaries, what properties must be
possessed by a Being including necessary
existence. To wit:-

I. It must be eternal.
For if a definite duration
be assigned to
it, it would beyond that definite
duration be conceived as non-existent, or as not involving necessary
existence, which would
be contrary to its definition.

III. It cannot be conceived as
determinate, but only as
infinite.
For, if the nature of the said Being were
determinate,
and conceived as determinate, that nature
would beyond the said limits be conceived as non-existent, which again is contrary to its definition.

IV. It is indivisible. For if it were divisible, it could be
divided into parts, either of the same or of different
nature. If the latter, it could be destroyed and so not
exist, which is contrary to its definition; if the former,
each part would in itself include necessary
existence,
and thus one part could exist without others, and
consequently be conceived as so existing. Hence the
nature of the Being would be comprehended as
finite,
which, by what has been said, is contrary to its
definition. Thus we see that, in attempting to ascribe to
such a Being any imperfection, we straightway fall into
contradictions. For, whether the imperfection which we
wish to assign to the said Being be situate in any defect,
or in limitations possessed by its nature, or in any
change which it might, through deficiency of power,
undergo from external causes, we are always brought
back to the contradiction, that a nature which involves
necessary existence,
does not exist, or does not
necessarily exist. I conclude, therefore-

VI. Further, since only from
perfection can it come
about, that any Being should exist by its own sufficiency
and force, it follows that, if we assume a Being to exist
by its own nature, but not to express all perfections, we
must further suppose that another Being exists, which
does comprehend in itself all perfections. For, if the less
powerful Being exists by its own sufficiency, how much
more must the more powerful so exist?

Lastly, to deal with the question, I affirm that there can
only be a single Being, of which the
existence belongs
to its nature; such a Being which possesses in itself all
perfections I will call
God. If there be any Being to
whose nature existence
belongs, such a Being can
contain in itself no imperfection, but must (by my fifth
premise) express every
perfection; therefore, the nature
of such a Being seems to belong to
God (whose
existence
we are bound to affirm by Premiss VI.),
inasmuch as He has in Himself all
perfections
and no imperfections. Nor can it exist externally to
God.
For if, externally to God, there existed one and the same
nature involving necessary
existence, such nature would be
twofold; but this, by what we have just shown, is absurd.
Therefore there is nothing save
God, but there is a single God,
that involves necessary
existence, which was to be proved.